Thats the first sign of decay, your child is developing "early childhood caries". You need to visit a pediatric dentist at the earliest for a proper examination. At this stage the decay can be arrested. The pediatric dentist will counsel you regarding diet and oral hygiene habits. If you ignore it at this stage, it will rapidly spread to all the other teeth as and when they erupt into the mouth.
Next Steps
visit a pediatric dentist
Health Tips
brush the teeth twice daily with a rice grain smear of toothpaste.
clean the teeth with clean cotton dipped in water after each feed/meal.